Sustained deep-water formation in the Nordic Seas during Marine Isotope Stages 5 and 4 and implications for carbon storage in the North Atlantic

Changes in the formation of North Atlantic Deep Water (NADW) and the expansion of southern-sourced waters in the Atlantic Ocean are linked to enhanced marine carbon storage during glacial and stadial periods, explaining late Pleistocene atmospheric CO₂ variations. However, the role of deep-water formation in the Nordic Seas, a key NADW source, and its influence on Atlantic overturning remains unclear, especially after the last glacial maximum. In this study, we present high-resolution reconstructions of bottom water [CO₃^2-] from Cibicidoides wuellerstorfi, along with stable isotopes and aragonitic pteropod abundances in marine sediment core PS1243 from the deep Norwegian Sea, to explore past deep-water dynamics and their impact on carbon cycling. Our data suggest continuous formation of dense, well-ventilated deep waters during Marine Isotope Stages 5 and 4, with a deepening of the aragonite compensation depth during the MIS 5b-to-4 transition. MIS 5e indicates resilience of Nordic Seas overturning in spite of a warmer North Atlantic and suggested summer Arctic sea ice reduction. A compilation of Atlantic [CO₃^2-] records suggests that dense waters from the Nordic Seas expanded into the western North Atlantic, reducing its carbon storage capacity during MIS 4 and stadial MIS 5. Our study highlights differences in the sensitivity of Atlantic and Nordic Seas overturning to past climate conditions, with implications for the Atlantic's role in atmospheric CO₂ variations.